I've noticed that interface, too and it almost reads too good to be true...
57 dB is not much for a Shure SM7b on quiet sources like speach, but will work in a rock song context with a strong voice.
To be safe on acoustic sources with a dynamic mic (including ribbons) 70-80 dB are nice to have, but not very common)

After all it depends on the signal to noise ratio and there's the catch in Tascam's specs:
it's listed as 101 dB or higher (mic/line in to line out, gain knob at minimum, JEITA)
Usually this figure doesn't increase linear with gain, so what you'll get with gain set at 80% will only be revealed on a real world signal.

I have 2 interfaces: the one with 5db better specs delivers a 10db higher (=worse) noise level - measured.
Most preamps show a significant increase of noise in the very final range of the gain dial.
So you may rather end with a usable 54dB of gain if 57dB are max value.
The -125dB equivalent input noise for the Tascam preamp doesn't tell much because the following stages are ignored.
They measured SNR at minimum gain setting for a reason

Awesome update, thanks! Amazingly it looks like that since the tascam iXR is MFi (Made For iPhone), you can use the regular charging cable to plug into it, and not have to fork over 30 bucks for the Lightning to Camera adapter!